1. Field of the Invention
The present invention relates to asymmetric calixarene derivatives and a process for the preparation of the same.
2. Description of the Prior Art
It has been found by C. D. Gutsche et al. that the reaction of a phenol with formaldehyde under suitable conditions gives cyclic tetrameric to octameric compounds, i.e., calixarenes (see, for example, J. Org. Chem., 43, 4905 (1978)). The benzene units of the calixarene each have such a freedom of causing conformational changes that they generally rotate at relatively high rates around room temperatures (see C. D. Gutsche et al., J. Am. Chem. Soc., 104, 2652 (1982)). Owing to this freedom, a calixarene is present as several conformational isomers. For example, a calix[4]arene is present as four conformational isomers, i.e., cone, partial cone, 1,2-alternate and 1,3-alternate. A calix[4]arene or a calix[5]arene generally takes a stable cone conformation owing to the rotation of the benzene units.
Meanwhile, it has been thought that a suitable derivative of a calixarene can possess an asymmetric structure. For example a calix[4]arene having at least three kinds of different benzene units, shown in FIG. 1, or at least one laterally unsymmetrical benzene unit, shown in FIG. 2, can possess an asymmetric strtucture. However, since such a calix[4]arene causes inversion owing to the rotation of its benzene units to give a racemization, the rotation of the benzene units must be hindered in order to obtain an optically active substance. It has been already known that the rotation of the benzene units of a calixarene can be hindered by converting a calixarene into a suitable derivative (see, for example, C. D. Gutsche et al., Tetrahedron, 39, 409 (1983)). However, when such a derivative is prepared by the process of the prior art, a mixture of various conformational isomers as described above is formed because of the hindrance to the rotation of the benzene units, and the isolation of a pair of optical isomers from the crude product is very difficult. Thus, neither a process for the effective preparation of a racemic modification of a calixarene derivative nor a process for the optical resolution thereof have been found as yet.